A frequent occurrence during the treatment of breast tumors that originally respond to hormonal therapy with antiestrogens is the outgrowth of resistant populations of cells having a more aggressive malignant phenotype. Two approaches will be used to investigate the genetic mechanisms underlying this form of breast tumor progression. The first approach uses cDNA libraries constructed in a novel Epstein-Barr virus-based shuttle vector to transfect and efficiently retrieve cDNAs whose vector-driven expression can confer a hormoneindependent phenotype to cells that are originally hormone-dependent. This is a new approach to finding which of the many genes that are differentially-regulated in hormone-responsive and hormone-independent cells or which of the many genes that are induced by estrogen treatment are the key regulatory genes that are responsible for the promotion of cellular growth. Preliminary results Indicate that this approach is indeed capable of finding a regulatory gene involved in growth factor secretion. This result will be confirmed and the expression vector will be refined in a manner that will allow its efficient use in in vitro and in vivo selections schemes aimed at isolating genes responsible for the hormoneindependent growth of breast cancer cells. The second approach is based on the hypothesis that estrogen-independence results from the acquisition of the ability to constitutively produce growth factors that are normally induced by circulating estrogens in estrogen-dependent tumors. These growth factors can have an autocrine and/or paracrine function in stimulating tumor growth. A newly identified member of the fibroblast growth factor family found in the conditioned medium of an estrogen-independent cell line will be purified and molecularly cloned. The biological relevance of production of this growth factor will be assessed by examining the range and extent of expression of the gene in hormonedependent and hormone-independent breast cancer cell lines and primary tumor tissues. Through transfection studies, it will be determined whether constitutive expression of the gene in a hormone-responsive cell line can confer estrogen-independence either in vitro or in vivo.